Light-Emitting Diodes or “LEDs” are increasingly being used for general lighting purposes. For example, LEDs are suitable for backlighting for LCD televisions, lightweight laptop displays, and light source for DLP projectors. Screens for televisions and computer displays can be made increasingly thin using LEDs for backlighting. In LED backlights, multiple strings of LEDs are arranged in parallel, and each string of LEDs has series-connected LEDs. To achieve good quality backlighting, various controllers are used to regulate the currents flowing across the multiple strings of LEDs.
FIG. 1 (Prior Art) is a diagram of a multiple LED string driver 10 comprising a boost converter 11 that drives multiple strings of LEDs via resistor ballasting. Boost converter 11 is driven by a feedback signal 14 across a resistor 16 that senses the current through one of the LED string 15. The output voltage VOUT of boost converter 11 is regulated to provide the necessary current. For the other LED strings, each has an identical resistor so that the current flowing through all LED strings are approximately the same. The variation of the LED string current, however, depends on how the LED forward voltages and the feedback voltage are matched. For example, if the total forward voltages of two LED strings are different by 1V, and the feedback voltage is 2V, then the mismatch in LED string current is 1V/2V=50%.
FIG. 2 (Prior Art) is a diagram of a multiple LED string driver 20 comprising an LED bias controller 21 that drives multiple strings of LEDs, each biased separately by a current sync. The current syncs are inside controller 21 and coupled to terminals CTRL1-CTRL6 (22-27) of controller 21. A power converter provides a regulated output voltage VOUT to the top of the LED strings, and the LED string current is each regulated by the current syncs. For best efficiency, the power converter output voltage VOUT is adaptively regulated so that only a necessary working voltage is dropped across the current syncs. The advantage of this approach is the LED string currents have high matching to each other. The disadvantage is that the total forward voltage variation from string to string is significant. As a result, the voltages across the current syncs vary, resulting in significant power loss and heat generation. For example, if the total forward voltages for two long LED strings are 200V and 180V respectively, then there is an additional 20V voltage drop across the current sync for the 180V forward voltage LED string. At 120 mA bias current, such a voltage drop results in an additional 2.4 W higher dissipation on the second LED string than the first LED string.
FIG. 3 (Prior Art) is a diagram of a multiple LED string driver 30 comprising a DC-to-DC controller 31 that drives multiple strings of LEDs. A boost converter 32 converts a 24V input DC voltage to a regulated output DC voltage VOUT (e.g., ˜100-200V) to the top of the LED strings. Each LED string bottom is separately driven by an LED string switching converter. Each LED string switching converter (e.g., switching converter 35), comprises a MOSFFET 36, an inductor 37, a diode rectifier 38, and a current sense resistor 39. Each LED switching converter individually operates like a buck converter, reducing the main output voltage to match the LED string total forward voltage so that each LED string current is regulated to a target value. As a result, there is no power loss caused by the voltage difference between the main output voltage and the LED string total forward voltage. However, because each LED string needs a separate switching converter having a separate inductor, the overall cost is high.